1. Field of the Invention
This invention relates generally to a focus control system and, more particularly, to a focus control system for an optical record carrier in which a light beam for recording or reproducing must remain focused on the carrier in spite of the fluctuation of the carrier.
2. Description of the Prior Art
It has been proposed in the art to record and reproduce signals optically, for example, to record a video signal on a flat record carrier and to reproduce a video signal previously recorded on such a carrier, the recording and the reproduction each being accomplished by means of a light beam scanning the surface of the flat record carrier.
In a system for such an optical recording or reproducing as mentioned above, it is required, for recording or reproducing signals accurately, that the light beam which scans the record carrier always be in the proper focus condition, for example, the condition of being in focus on the surface of the record carrier without being influenced by deviations of the carrier from a predetermined position.
However, if a rotatable disc is employed as the flat record carrier and its surface is used for recording signals as usual, and if the surface of the rotating disc is inclined in relation to its axis of rotation or if the surface of the rotating disc moves along the axis for rotation through the influence of the rotation of the disc, even though such an inclination or movement is usually present to a certain degree in practical systems, the space between the surface of the disc and an objective lens provided for focusing the scanning light beam on the surface of the disc would vary, which is undesirable. As a result, the scanning light beam can not always maintain the proper focus condition on the surface of the disc.
To compensate for deteriorations in the focus condition of the scanning light beam due to the causes just described, it has been proposed in the prior art systems to detect the instantaneous position of the surface of the rotating disc and, in response to the detected information, to move the lens to maintain a predetermined space between the surface of the disc and the lens so as to keep the scanning light beam always in the proper focus condition on the surface of the rotating disc. However, it is difficult for the servomechanism of the proposed system to follow quick variations in the space with fidelity because the objective lens whose mass is relatively large can not be moved quickly and accurately enough. Therefore, the compensation is not done adequately.
There has been another proposal to keep the space between the objective lens and the carrier constant: It is to run the carrier between sliders in order to stabilize the fluctuation of the carrier aero-dynamically. However, it is impossible to eliminate fluctuations of the carrier completely.
Another previous proposal to avoid the deteriorations in the focus condition of the scanning light beam is to install the rotatable disc so that its surface is perfectly perpendicular to its axis of rotation and to prevent the surface of the rotating disc from having any movement in the direction of the axis for rotation. This proposal, however, is impossible to obtain in any practical system.
Accordingly, the prior art systems have not accomplished sufficient compensation or avoidance of the deteriorations in the focus condition of the light beam scanning the rotating disc.